Palestinian militants launched hundreds of rockets into Israeli territory, targeting Tel Aviv for the first time, and Israel intensified its aerial assaults and sent tanks rumbling toward the Gaza border for a possible invasion.

Defense Minister Ehud Barak of Israel expressed outrage over two long-range Palestinian rockets that whizzed toward Tel Aviv and triggered the first air raid warning in the Israeli metropolis since it was threatened by Iraqi Scuds in the 1991 Persian Gulf War.

"There will be a price for that escalation that the other side will have to pay," Barak said.

He authorized the call-up of 30,000 army reservists if needed, another sign that Israel was preparing to invade Gaza.

An invasion would be the second in four years designed to crush what Israel considers an unacceptable threat from smuggled rockets amassed by Hamas, the militant Islamist group that governs the coastal enclave and does not recognize Israel's right to exist.

It was not clear whether the show of Israeli force in fact portended an invasion or was meant as more of an intimidation tactic to further pressure Hamas leaders, who were forced into hiding Wednesday after the Israelis killed the group's military chief in an airstrike.

But Israeli Prime Minister Benjamin Netanyahu said he was prepared to "take whatever action is necessary.

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Although Tel Aviv was not hit and the rockets heading toward the city of 400,000 apparently fell harmlessly elsewhere, the ability of militants 40 miles away to fire them underscored, in the Israeli government's view, the justification for the intensive aerial assaults on hundreds of suspected rocket storage sites and other targets in Gaza.

Health officials in Gaza said at least 19 people, including five children and a pregnant teenager, were killed over two days of nearly nonstop aerial attacks by Israel, and dozens had been wounded.

Three Israelis were killed Thursday in Kiryat Malachi, this small southern Israeli town, when a rocket fired from Gaza struck their apartment house.

In Washington, Obama administration officials said they asked friendly Arab countries with ties to Hamas, which the U.S. and Israel regard as a terrorist group, to use their influence to seek a way to defuse the hostilities.

At the same time, however, State Department spokesman Mark Toner reiterated to reporters the U.S. position that Israel had a right to defend itself and that the "onus was on Hamas" to stop the missile attacks.

There was no sign that either side was prepared to restore a truce that has mostly prevailed since the last time the Israelis invaded Gaza in the winter of 2008-09, a three-week war that left 1,400 Palestinians dead and drew international condemnation.

Denunciations of Israel for what critics called a renewal of its aggressive and disproportionate attacks spread on the second day of the strikes. The biggest criticism came from the 120-nation Nonaligned Movement, the largest bloc at the United Nations.

Iran, which holds the group's rotating presidency, said: "Israel, the occupying power, is, once more, escalating its military campaign against the Palestinian people, particularly in the Gaza Strip."

The group made no mention of the Palestinian rocket fire and demanded "decisive action by the U.N. Security Council."

The Israeli military said it attacked 70 underground rocket-launching sites in Gaza, and "direct hits were confirmed."

There were also unconfirmed reports that Israeli rockets had struck near Gaza's Rafah crossing into Egypt, forcing its closing.

Brig. Gen. Yoav Mordechai, a military spokesman, said the airstrikes hit more than 300 sites in Gaza by late Thursday, and "we'll continue tonight and tomorrow."

He said militants in Gaza fired more than 300 rockets into southern Israel and at least 130 more had been intercepted by Israel's antimissile defense system.

Southern Israel had been the target of more than 750 rockets fired from Gaza this year that hit homes and caused injuries.